Massive choice out there and difficult to recommend any particular type without considering the 'thrill factor' expectations of the riders. Good news is they're all fun.
Decent selection at Boatworld with good descriptions and some with videos to give an idea of ride.
We only tow behind a small boat and have a Jobe Hotseat ringo for one - easy to sit in and stable ride, not really steerable.
For a bit more fun we have an Airhead Slash 2 for one or two riders and is steerable.
To keep it flexible probably best to avoid anything with side by side seating. The larger ringos, deck tubes and smaller banana tubes can all be used by one or two riders. Some designs have lower profile at the back to make it easier to get back on from the water - good for kids and tired adults!